Now user can't be edited to have same username as another user. Also fixed username validity logic and added tests.

Added info for session.
Modified user edit post invalid username handling to match creation invalid username handling.
Refactored user edit post error handling.
Removed dot.
Added NotAuthenticated exception, and mapped it to redirect. Now base model building throws NotAuthenticatedException instead of SerciceException, if session is not found. Now UserResource ServiceExceptions and RenderExceptions are handled with redirects.